~ The Power of Numeracy

Monthly Archives: August 2009

Having now submitted a review of the empirical support for weakening of the Walker circulation to the Journal of Geophysical Research, I can get back to the “Is the sea level rise accelerating?” article. The abstract is below:

No Significant Basis for Acceleration in Sea level Rise Last Century

Abstract: Various authors have claimed that sea level rise is accelerating. This assumption is responsible for the high end projections for sea level by the year 2100. We evaluate three main datasets, and two studies, finding no support for an acceleration term in models fit to sea level over the 20th century, and a significant deceleration in satellite altimeter readings from 1993. The only empirical basis for sea-level acceleration arises from the inclusion of data prior to 1900, which cannot be attributed to the influence of greenhouse gases. Moreover, core references for projected sea level rise contain errors and imprecision that bias upward projections for sea level in 2100. We conclude that acceleration of sea level rise both in observations and the projections of climate models cannot be substantiated at this time. We find broad justification and agreement for empirical linear models, greatly reducing CI projections based on linear extrapolation.

Below is the abstract of the manuscript I have been preparing. A draft is available via the contact form above if you are interested in helping out with feedback. Comments from the mysterious Dr Jones that prompted this manuscript are listed below.

Recent Data Show No Weakening of the Walker
Circulation
David R.B. Stockwell and Anthony Cox

Abstract: Various authors have examined the strength of the equatorial Pacific overturning known as the Walker Circulation in both climate models and observations, attributing a generalized weakening to anthropogenic global warming. Here we review the analysis in Power and Smith [2007] using updated Southern Oscillation Index (SOI) and NINO sea surface temperature indices. We find no significant long-term changes in the indices, although the SOI appears to have recovered from an anomalously low period from 1976 to 1998. The increasing sea surface temperature in the NINO4 region is not significant, nor representative of other NINO regions. The findings of a weakening Walker circulation appear to be premature, and the corresponding climate model projections cannot be substantiated at this time. The reports of weakening of horizontal atmospheric circulation in climate models should be regarded as an inconsistency and not as an indicator of anthropogenic climate change.

The El NiÃ±o pattern across the Pacific has not intensified during the past fortnight. Furthermore, the coupling between the ocean and atmosphere which amplifies and maintains El NiÃ±o events has so far failed to eventuate. The neutral SOI and sub-surface cooling are evidence of this.

However, the Trade Winds are weakening over a broad area and this may promote renewed warming. In addition, leading climate models continue to predict further development of the El NiÃ±o, although not as emphatically as a month or two back. Therefore, the odds remain strongly in favour of 2009 being recognised as an El NiÃ±o year.

We also document what appears to be a concurrent period of unprecedented El Nino dominance. However, our results, together with results from climate models forced with increasing greenhouse gas levels, suggest that the recent apparent dominance might instead reflect a shift to a lower mean SOI value.

The idea is that global warming is causing a weakening of the Walker circulation, and consequently more negative SOI values, on average. Dr Jones has proposed that this is the reason the cumulative sum of SOI is highly correlated with global temperatures. I think the data only show the ‘period of unprecedented El Nino dominance’ between 1976 and 1998 produces the trend, and cSOI largely matches all the wiggles in between.

Perhaps the difference is: a belief in a generalized weakening in the Walker circulation related to GW, vs a specific anomalous period of SOI generating a large part of global warming.

Power and Smith go on to make some other interesting statements. They note the disagreement about ENSO between models, and the relationship between weakening of the Walker circulation and the apparent intensity of the post 1977 period:

[19] However, if global warming is largely responsible for the observed decline in the average value of the SOI over the period 1977 â€“ 2006 then the threshold values used to define ENSO events need to be lowered (by approximately 3 SOI units). Under the new thresholds the apparent dominance of El Nino disappears.

This is similar situation whereby the frequency of extreme events, like heatwaves, appear to increase when the trend of average temperatures is increasing. Finally they say:

This simple interpretation gives a result that is consistent with modelling results:
global warming weakens the Walker Circulation and warms the tropical Pacific Ocean, but has little impact on tropical ENSO-driven variability about the new mean-state [Meehl et al., 2007]. While plausible, further research is needed to help quantify the extent to which global warming has in fact driven the unprecedented recent decline in the 30-year average value of the SOI. Southern Oscillation Index (SOI, blue).

However, they did not exclud the possibility that the apparent drift in Walker circulation is an artifact of the ‘period of unprecedented El Nino dominance’. They support the change in the baseline SOI with a structural break model, a methodology that luke claims is not used by real climate scientists and Nick says is ‘technical analysis’.

The break model is consistent with a ‘period of unprecedented El Nino dominance’, not only a weakening in the Walker circulation. To support their hypotheses they would need a significant negative slope on the SOI and other indexes prior to 1977, something that I bet isn’t there — a little fly in the ointment not disclosed by the authors.

It does not affect our study however, as the presence of a ‘drift down’ in the SOI mean would not affect the correlation of the cSOI, as linear regression adjusts the coefficient to compensate for this. This is one of the reasons its unreliable to estimate relative contributions from linear regressions.

Nor does it affect attempts to quantify the effects of the ‘period of unprecedented El Nino dominance’ on global mean temperature — an effect despite acknowledgment in the AR4 does not have been seriously considered as a contributor to the ‘unprecedented’ warming trend since 1960.

Dr Jones drew my attention to a paper by Vecchi (2006) that he feels rebuts the view that increased frequency and intensity of El Nino events are responsible for global warming and not increasing greenhouse gases — so I give a quick analysis of it here.

Dr Jones mentions a confusion cause and effect, so let’s try to clarify the different positions first. The two views are something like this for global temperature (GT) and Walker circulation (WC):

The primary evidence for H2 is the weakening of the delta sea level pressure between the east and west tropical Pacific (ΔSLP), representing weakening of the Walker circulation as a response to increasing GHGs. The Figure 3 from Vecchi that is supposed to supply this evidence is below.

The declining slope of the linear fit to the ΔSLP is supposed to prove that the Walker circulation has been decreasing over the last century.

The first thing I notice is that the negative slope is almost certainly due to the period I have marked with red lines between 1976 and 1998 — the period between the Great Pacific Climate Shift in 1976 and the potential shift we identified in 1998 in our ‘breaks’ paper. For comparison, below is an image of PDO over the period, showing that 1976-1998 corresponds to a warm phase of the PDO.

Here also is our ‘break’ figure with the statistically significant breaks at 1976 and 1998 indicated.

Note the coincidence between the period of greatest increase in temperature, period of warm PDO, and low ΔSLP. Outside that period: nada, zilch. ΔSLP seems to have returned to normal after 1998, further contradicting the projections in Vecchi that the decline in the strength of the Walker circulation will continue to 2100.

If so, this paper will be easy to discredit. Simply update the ΔSLP to 2009, and show that the downtrend is simply due to a single anomalous period of high El Nino activity. That the GCM predicts a decline in the Walker circulation — which does not occur in reality — is further evidence of unreliability of GCM’s in this area.

Here is the abstract for our comment submitted to Geophysical Research Letters today. Bob Tisdale is acknowledged as the source of the idea in the first paragraph. Lets see how it goes. If you would like a copy, contact me via the form above.

Comment on “Influence of the Southern Oscillation on tropospheric temperature” by J. D. McLean, C. R. de Freitas, and R. M. Carter

David R.B. Stockwell and Anthony Cox

Abstract

We demonstrate an alternative correlation between the El Nino Southern Oscillation (ENSO) and global temperature variation to that shown by McLean et al. [2009]. We show 52% of the variation in RATPAC-A tropospheric temperature (and 59% of HadCRUT3) is explained by a novel cumulative Southern Oscillation Index (cSOI) term in a simple linear regression model and 65% of RATPAC-A variation (67% of HadCRUT3) when volcanic and solar effect terms are included. We review evidence from physical and statistical research in support of the hypothesis that accumulation of the effects of ENSO can produce natural multi-decadal warming trends. Although it is not possible to reliably determine the relative contribution of anthropogenic forcing and SOI accumulation from multiple regression models due to collinearity, these results suggest a residual accumulation of around 5 Â± 1% and up to 9 Â± 2% of ENSO-events has contributed to the global temperature trend.

While any error in science is important, and those identified should be swiftly corrected, my concern has always been non-trivial errors of statistics. My beef is that large tranches of AGW orthodoxy are supported by claims that do not pass standard tests of significance. Why is significance important? Wikipedia states that a result is called statistically significant if it is unlikely to have occurred by chance. Therefore many of the claims may be simple change occurrences that scientists are being fooled into believing by their own prejudices. For example:

Last week I prepared a comment defending McLean’s SOI paper. I shall send it in shortly. Basically, I extend their analysis a little and show that the majority of variation in a linear regression model predicting global temperature (not differences) can be accounted for using SOI-related terms.